Multi-Currency: add processing code to PaymentsToSalesforce GW for multi-currency support

[greenstork]

In order to fully support multi-currency Salesforce organizations, we need to add to the processing class, the Currency Code variable, lookup to the currency table which is enabled in multi-currency orgs, setting of the CurrencyISOCode on the appropriate objects (Opportunity, Payment Notification, Payment).

[greenstork]

COnfiguration documentation would also need to be updated here in the Readme